Measurements of nerve conduction velocity in peripheral nerves has long been a valuable diagnostic tool in orthopedic surgery, neurology and other branches of medicine. It is particularly useful in the diagnoses of various nerve compression syndromes. A diminished velocity is abnormal, and suggests nerve damage which may require immediate surgical intervention.
The traditional technique for making nerve conduction velocity measurements is by stimulating the peripheral nerve with an electrical impulse and measuring the elapsed time from the stimulus until an action potential occurs in a muscle innervated by the nerve under examination. By repeating this technique using a more distal stimulation point along the course of the nerve, and by knowing the distance between the two points of stimulation, the nerve conduction velocity can be calculated. In practice, the measurements are made by the use of surface electrodes positioned over the muscle that pick up the signals which are then amplified and displayed on the screen of an oscilloscope. Measurements of the distance between the stimulus and response peaks on the screen of the oscilloscope are then converted to latency times. This established technique for making such measurements is known as electromyography (EMG). EMG is quite accurate, but requires sophisticated equipment and specialized training to operate the equipment. Thus, such tests are quite expensive and require the inconvenience of moving the patient to the place where the equipment is located. Although much information about the nerve's operation is thus obtained, it cannot be used with any frequency nor can it conveniently be used in emergency situations.
There are numerous medical situations in which frequent monitoring of nerve conduction velocities would be of extreme value to the physician, particularly if such measurements could be made at bedside. For example, where trauma to an extremity has caused pressure in a constrictive fascial compartment, progressive irreversible damage to nerves and muscles can result from ischemia unless immediate surgical decompression takes place. Swelling in a plaster cast can cause a similar problem. Current methods of measuring compartment syndrome involve the introduction of wick catheters into the compartment to measure tissue pressures, but these procedures are invasive and frequently unreliable. Since nerve conduction velocity decreases long before permanent damage results from a compartment syndrome, a method of quickly and easily determining the nerve conduction velocity would be of great value in aiding the physician in making crucial decisions in such cases. There is therefore a need for a portable, simple and inexpensive device for measuring nerve conduction velocities on a frequent basis to provide a useful objective parameter of the patient's clinical progress.